CME Slides Forum - M. Fukagawa

A joint Congress by ERA-EDTA and ISN

FGF23 AND KLOTHO IN PATIENTS WITH CKD

Masafumi Fukagawa, Kobe, Japan

Chair: Pascal Houillier, Paris, France

Heini Murer, Zurich, Switzerland

fukagawa

Dr. M. Fukagawa
Division Nephrology and Kidney Center
Kobe University School of Medicine
Kobe, Japan

Good morning, thank you Chairman and members and guests. Between two basic scientists I would like to present a short talk on clinical hot topics. So my talk will be on FGF23 in patients with CKD especially with ESRD.

As all of you know, FGF23 is a phosphaturic hormone which is secreted from the bone in response to dietary phosphorous load, as well as to the increase of serum phosphate levels.

In an animal model with renal dysfunction the serum FGF23 levels change in response in this slide to the sevelamer treatment very rapidly and returns to normal. A brief look at this one, 1% of sevelamer and this is 3% of sevelamer. So a decrease of FGF23 is very significant with higher doses of sevelamer but this is with animals with renal dysfunction.

So in general it has been quite difficult to show, to demonstrate a regulation of FGF23 with normal kidney function because the serum phosphate level does not change so easily. This is one kind of answer by Ferrari and associates. They showed in normal humans that changes of FGF23 level correlated well with the 24 hour urinary phosphate excretion. Thus the body senses the changes of phosphate dietary intake through still unknown mechanisms and in response to this stimulus FGF23 is secreted from the bone.

A secondary hyperparathyroidism is the most popular abnormality of CKD-MBD. As shown in this slide, phosphorous retention stimulates directly FGF secretion and indirectly by causing hypocalcaemia or by suppressing the production of 1, 25 (OH) 2D3. FGF23 plays a role here in this point. I will show several data.

As we have shown in studies of a CKD patient, if their FGF23 levels decreased as the creatinine clearance decreased and especially it -- became less than 50%.

TmP/GFR correlated very well in patients with relatively good kidney function but this correlation disappeared in patients with reduced renal function. This is also the same in the negative correlation between FGF23 and 1, 25 (OH)2 D3 as can be seen. In patients with relatively good renal function the correlation was significant but it disappeared in patients with very low kidney function.

So in summary, with normal kidney function FGF23 acts on the kidney to efficiently excrete -- phosphorous without affecting parathyroid function. By contrast when the number of -- nephrons decreases in patients with progression of CKD then at this stage despite high FGF23 levels, the amount of net phosphate excretion does not increase sufficiently and serum phosphate levels remain high. High phosphate levels as well as decreased 1, 25 (OH)2 D3 level stimulates Pi excretion leading to the development of secondary hyperparathyroidism in the late stages of CKD.

A recent interesting topic in predialysis CKD patients is that high PTH levels may predict progression of CKD, as shown in this slide with FGF23 or intact FGF23.

However, at the moment its pathophysiological significance remains to be illustrated.

Then what is the role of regulation of FGF23 in dialysis patients with just the main target organ the kidney?

In patients with longer than 10 years of dialysis -- that means no registered renal function, serum FGF23 levels were extremely high, more than 100 or thousand times compared to predialysis patients.

But why? Why is it so high in these dialysis patients and so, different among the dialysis patients? One possibility is accumulation of fragments but this is not the case because we used a kit that exclusively detects the whole molecule.

So then what are the stimuli for FGF secretion in these dialysis patients? We are wondering whether high levels of FGF23 really have the effect to target organs.

In this unpublished study we checked the changes of average dietary phosphorous intake with changes of FGF23 levels and we found a positive correlation and together with other data from this study we can conclude that even in dialysis patients dietary phosphorus load is a major stimulus for FGF23 production. As for the PTH, we have found a very interesting phenomenon, PTH showed a weak correlation but to our interest it correlated better to the PTH level at 2 years after the detection of FGF23.

So we analysed 103 non-diabetic dialysis patients. PTH levels at the time of FGF measurement were below 300 in all patients. After 2 years of treatment by nephrologists intact PTH levels exceeded 300 only in 17 out of these patients. As shown in this slide, initial PTH levels were comparable between controlled and uncontrolled groups but as can be seen on the left, the FGF levels at the start of the observation were much higher in the uncontrolled group compared to the controlled group

and analysis of the ROC study revealed that the measurement of serum FGF23 level was the best screening test to discriminate patients in whom advanced secondary hyperparathyroidism develops within two years. We have the same kind of data in patients with already established hyperparathyroidism. Their response could also be predicted by the serum FGF23 level at the start of the treatment.

Then where was the origin of such high levels of FGF23?
Of course, it is from the bone and not from the very enlarged parathyroid. Then what are the mechanisms of these very high levels of FGF23? Because at that time it had been shown that 1, 25 (OH)2D3 increases FGF23 production in vivo and in vitro.

We next examined the changes of FGF23 levels during calcitriol treatment. As shown in this slide, FGF23 levels progressively increased throughout the 6 months of calcitriol treatment

and FGF23 levels at 6 months of the treatment were significantly correlated with the cumulative doses of calcitriol.

So in summary, serum FGF23 levels are extremely high in dialysis patients without low kidney function. In these patients severe hyperparathyroidism and high levels of serum phosphorous, as well as dietary phosphorous load stimulate the production of FGF23 in the bone. In addition, 1, 25 (OH)2D3 including exogenous vitamin D analogs further increase FGF23 levels.

We are not sure whether these high levels of FGF23 have any effect on the parathyroid at the time we collected this data but concerning this data Doctor Silver’s group has recently shown that FGF23 suppresses parathyroid function through FGF receptor and klotho.

So our next question is whether very high levels of FGF23 can really decrease or suppress parathyroid function or if there is any possible resistance to parathyroid of FGF in uraemia?

Since several laboratories are trying to directly demonstrate the attenuated action of FGF23 on parathyroid in CKD we have selected another approach. In CKD patients marked decrease of klotho has already been reported at least in the remaining kidney, thus we next examined the expression of FGFR

This is an example of the staining for FGFR1 and this is a normal parathyroid and this is a parathyroid from dialysis patients as can be seen. The expression is significantly downregulated and also the klotho expression was also downregulated compared to the normal parathyroid gland.
Since we can see the difference between the part of diffuse hyperplasia and nodular hyperplasia, we next scored the staining. As can be seen here, as with vitamin D receptor or a calcium sensing receptor, the expression of klotho and FGFR1 was severely downregulated in nodular hyperplasia.

So in summary, the expression of both FGFR1 and klotho were reduced in hyperplastic parathyroid glands in uremic patients and this furthermore the expression was lower in nodular hyperplasia. This decreased expression may in part explain the resistance to FGF23 in uremic patients if present.

The final topic is the role of FGF23 on the survival of dialysis patients. VDRA treatment has been shown to be beneficial for survival and on the other hand VDRA treatment increases serum levels of phosphate and FGF23. In addition, recently Woolf and associates have reported that high FGF23 levels are associated with poor survival.

To me these results are inconsistent, so at the moment I cannot find any explanation for this inconsistency because this risk of FGF23 was also shown in patients without hyperphosphatemia, so it should be an independent risk factor.

So at the moment I have no idea to explain this phenomenon but this is a hypothesis recently suggested by Razzaque and associates he suggested that if this very high level of FGF23 may have some action on the tissue without klotho, there is no proof for this hypothesis because we don’t have enough information on FGF23 action but this is a very attractive idea.

So in all summary dietary phosphorous load as well as high serum levels increase serum FGF23 levels. Treatment by VDRA further increases serum FGF23 levels. It remains unclear whether such high levels of FGF23 suppress or increase parathyroid function. Thank you for your attention.

Question: Like you I’m a little confused in regards to the action of FGF23 on parathyroid hormone secretion. I was very impressed with the data from Justin showing suppression. In most of your talk you didn’t feel in the same way. Then we have done preliminary work in vitro and we incubated parathyroid cells with different concentrations of FGF23. We did not find any increase or decrease. This is only preliminary work, we need to now do much more work with different concentrations of FGF23 but we were hoping to find a suppression but we didn’t find anything, also there was no stimulation. Preliminary work, only preliminary.

Dr. Fukagawa: This is a very controversial area so at least I think that if very high levels of FGF23 act on the parathyroid --- uraemia that effect are not enough so we should show that a direct effect is not enough to suppress the parathyroid function in these advanced cases of CKD.